My iPad2 runs worse and worse with every new version of iOS. I wish I could go back to 6.
On one hand, nobody wants to buy a shiny new device and find that the software isn't taking advantage of the hardware you paid for in order to preserve performance on older models. On the other hand, the more rapid pace of mobile hardware development and the inability to add inexpensive upgrades can make it seem like you have little choice but to replace the things every couple of years.
It doesn't help that there's no official way to downgrade if your experience is unsatisfactory (no reinstalling XP on that old beater PC because it doesn't have the RAM to run Vista well, etc) and applications quickly start to require the latest version of the OS for program updates.